Episode 15: 3D stretchable electronics achieves ultrahigh conductivity

Prachi Patel of MRS Bulletin interviews Benjamin C.-K. Tee of the National University of Singapore about an interfacial design for stretchable electronics that uses three-dimensional helical copper micro-interconnects embedded in an elastic rubber substrate. Read the article in APL Materials.
Transcript
PRACHI PATEL: Metals are excellent at conducting electricity but not the best at being stretched or bent. For electronics that can be worn or wrapped around curved surfaces, stretchable conductors are key.  

BENJAMIN TEE: One good example is a smart patch that you can wear to record your heartbeat, or your ECG and so on. 

PATEL: That’s Benjamin Tee at the National University of Singapore. He and his colleagues have come up with a new way to make stretchable conductors that stay strong and remain highly conductive when stretched to almost twice their length. Their strategy overcomes two main challenges of previous stretchable conductors. 

TEE: So one approach to make stretchable conductors is to use nanomaterials like carbon nanotubes, graphene. These are one way where people use these particles and coat it onto a stretchable substrate like silicone rubber or polyurethanes.

PATEL: Another approach is to use metal thin films. Basically, researchers create wavy serpentine patterns of these films so they can stretch with the substrate. But in both approaches, stretching the materials tends to reduce their conductivity. Plus, the thin materials have lower electrical conductivities than bulk metal. So Tee and his colleagues took a different approach. 

TEE: We drew inspiration from actually spring-like structures. Spring-like structures are able to withstand strain. If you either stretch on a spring or compress a spring, they return, right? 

PATEL: They first made a spring using some off-the-shelf copper wire. Then they embedded it in silicone rubber to make it elastic. But that still wasn’t good enough. The spring started changing shape within the rubber after being stretched a few times.  

TEE: And we found out that the reason was that the interface between metal and rubber needs to be well-matched. If you’re talking about metals you have modulus is extremely high in the gigapascals range whereas rubbers typically have a modulus of megapascal range. There’s a three orders of magnitude difference. So we need a way to make sure that these two interface do not slip. 

PATEL: And they did that by adding an epoxy to the rubber, which helps bond the metal to the rubber. This did the trick. 

TEE: We can stretch it over a 1000 times and these springs stay in the same shape as they were after stretching. What’s interesting is that this electrical conductivity does not change because we’re not changing the crystalline structure of the metal. Our approach basically extends the dimension into 3D as opposed to a planar patterned film. We’re exploiting the bulk property of the metal. The other advantage is it can actually stretch more because we’re going into three dimensions. So I think there is certainly a limit to how much we can scale this down if we want to keep the same good electrical properties that we’re talking about. But that being said when you scale them down to about a micron, they actually become softer and so you can have even greater stretchability. So I think a micron or so is sort of where we want to be if you want to capture bulk properties and still retain the stretchability.

PATEL: The team’s findings are published in APL Materials. My name is Prachi Patel from the Materials Research Society.